It is known that an internal combustion engine of a motor vehicle is equipped with an aftertreatment system designed to change the composition of the exhaust gas in order to reduce the pollutant emissions. Some aftertreatment systems may include a catalytic converter, for example a Diesel oxidation catalyst (DOC), followed by a particulate filter, for example a Diesel particulate filter (DPF). A Diesel oxidation catalyst usually comprises a catalyst substrate (core), for example a ceramic monolith with a honeycomb structure, which supports a wash coat that carries catalytic materials, usually a mixture of precious metals, suitable for prompting the oxidation of unburned hydrocarbons (HC) and carbon monoxides (CO) into carbon dioxides (CO2) and water (H2O).
A Diesel particulate filter is a device that includes a ceramic body similar to the core of an oxidation catalyst but specifically designed to trap diesel particulate matter or soot contained in the exhaust gas. Most diesel particulate filters also include a wash coat containing a small quantity of precious metals, whose concentration is much lower than the concentration of precious metals in the oxidation catalyst (e.g. 10%) but still enough to promote oxidation reactions at high temperatures. When the accumulated particulate matter exceeds a predetermined threshold value, the diesel particulate filters are subjected to a regeneration process that empties the filter and restores its original efficiency.
This regeneration process is usually performed by increasing the temperature of the particulate filter up to a temperature (e.g. 630° C.) that causes the accumulated particulate matter to burn off. One of the most widely used strategies to increase the filter temperature is that of operating the fuel injectors of the internal combustion engine to execute so-called post injections. The post injections are small quantities of fuel that are injected into the combustion chambers of the engine during the exhaust stroke of the piston, when the exhaust valves are already open. These small quantities of fuel exit unburnt from the combustion chamber and reach the Diesel oxidation catalyst, where they are ignited and generate a stream of hot exhaust gas that flows towards the Diesel particulate filter located downstream, thereby increasing its temperature.
However, during the lifetime of the Diesel oxidation catalyst, the conversion efficiency of this component is not constant but decreases progressively due to ageing and/or poisoning effects. For this reason, it may happen that the conversion efficiency of the Diesel oxidation catalyst reaches a level which still allows this component to reduce the polluting emissions during the normal operation of the engine, but makes it unsuitable to efficiently oxidize the post injected fuel quantities during the regeneration process of the Diesel particulate filter. As a consequence, these post injected fuel quantities may exit unburnt from the Diesel oxidation catalyst and be ignited just inside the Diesel particulate filter, due to precious metals contained in its wash coat, thereby increasing the thermal stress of this component and of other neighboring components of the motor vehicle, such as the vehicle under hood and underfloor.